Lesson Plan #30         http://www.phy6.org/stargaze/Lgravity.htm

(20)  Newton's Theory of Universal Gravitation  

  This lesson re-derives Newton's result, that if the force of gravity decreases like the square of the distance from the center of attraction, the same force that causes objects on the Earth"s surface to fall can also be responsible for the motion of the Moon.

Part of a high school course on astronomy, Newtonian mechanics and spaceflight
by David P. Stern

This lesson plan supplements: "Newton's Theory of Universal Gravitation," section #20   http://www.phy6.org/stargaze/Sgravity.htm

"From Stargazers to Starships" home page: ....stargaze/Sintro.htm
Lesson plan home page and index:             ....stargaze/Lintro.htm

Goals: The student will learn

  • How Newton tied together the gravity observed on Earth and the motion of the Moon.

  • To apply an earlier lesson about centripetal forces.

  • To appreciate the philosophical implication of Newton's calculation: all parts of the universe seem to obey the same laws of nature.

    Terms: Gravitation (or gravity), inverse-square law, solar constant.

    Stories: The Story of Newton's apple is traced to its source. Also, mention is made of the black hole at the center of our galaxy, of Cavendish and Eötvös (and in this lesson plan, of Coulomb).

        Starting the lesson:

        Today we will learn about Newton's theory of universal gravitation. The story is often told that Newton was inspired to this by a falling apple--with some going so far as to suggest it bonked him on the head as he was sitting under a tree. We will come back to that story.

        Of course, Newton did not "discover" gravity. People in his time were well aware that the Earth attracted all objects near its surface, and Galileo, as you learned some time ago, experimented with falling objects. Other people studied the trajectories of cannonballs, which depend on gravity. But all these observations had to do with what happened close to Earth, in a region accessible to experimenters. The objects in the sky--planets orbiting in space, the Moon circling around the Earth--seemed to belong to a different class of phenomena, with separate rules, such as Kepler's laws. Astronomers were only beginning to understand those phenomena.

        Newton's achievement was in making the connection, in proposing that the same force that pulled apples down also pulled the Moon.

        Suppose, he said, that the force decreased like the square of the distance from the center of Earth. The Moon is 60 times more distant than the surface of the Earth, so the force of gravity should be 60 times 60, or 3600 times weaker. Would that equal the centripetal force mv2/R or 4π2N2R needed to hold the Moon in its orbit?

        Newton could calculate that--he knew R from Aristarchus and Hipparchus, and he knew N = 1/T, where T was the orbital period of the Moon. As you will see, the result fit well, and Newton concluded that it was indeed the gravity of the Earth that held the Moon to its orbit. He also guessed (correctly) that the Sun had a force of gravity much like the Earth's, and that was what kept the planets going around it. That is why he named his idea the theory of universal gravitation.

        Was the inverse squares law just a lucky guess? It was more. Imagine a source of light--say, the Sun--shining at equal intensity in all directions, sending out each second sunlight carrying an energy of I joules--that is, radiating a power of I watts. What is the density at which solar power flows at a distance of R meters, across an area of, say, 1 square meter (1 meter2)?

        A sphere of radius R centered on the Sun has an area 4πR2 meters2. Since all parts of the sphere receive equal illuminaton, each square meter receives a power I/4πR2 watts, and obviously, that intensity decreases like 1/R2. At Earth's orbit it equals 1360 watt/m2, a quantity known as the solar constant. At the orbit of Jupiter, R is about 5 times larger, and the light intensity is only 1/ 52 = 1/25 of the solar constant, which is why Pioneers 10 and 11, and Voyagers 1 and 2, preferred a power source other than solar cells. And although a lightbulb does not shine with equal intensity in all directions, your reading illumination, too, will decrease approximately like the square of the distance from it.

          [By the way: for historical reasons, phenomena which spread energy and obey the 1/R2 law are often classified as radiation. Microwaves used to heat food in the kitchen are called radiation, fast ions and electrons emitted by radioactive substances are also called radiation--even though the two phenomena are different and unrelated. Microwaves are similar to light and radio, ions and electrons are fast-flying pieces of matter.

          Students should be made aware that the damaging kinds of radiation are the ones also known as ionizing radiation, "ionizing" meaning they deliver enough energy to atoms and molecules to tear off electrons. These can be fast ions and electrons from radioactivity, or high-energy "electromagnetic" radiation similar to microwaves, radio waves and light, but of the types known as x-rays and gamma rays.]

        Newton was aware of such results, and the 1/R2 law was a natural assumption for him. Today that calculation is easy to repeat--but remember, one needed at least some idea of the laws of motion and of the centripetal force which they prescribe. Galileo and Kepler lacked that knowledge, and could not have tested the 1/R2 law without it. Newton also needed justification (which he found) for his claim that the gravity pull outside a sphere would not change if all the sphere's mass were concentrated at its center.

        Newton's "universal" gravitation led to a principle which today is all too easily taken for granted: that the same physical laws hold anywhere in the universe. All the astronomical evidence since Newton's time seems to confirm it: not just the laws of gravity, but the chemical elements, the speed of light, the way light is produced and other physical processes seem to be the same on Earth and in distant stars, or even in other galaxies.

    Now let us go back to that apple. (Continue with the original story of Newton's apple as given in "Stargazers", which a student--with clear voice--might read aloud in class).

    Questions to ask
      (Since the entire lesson focuses on a single calculation, not much can be asked beyond its details.)
    What idea did the falling apple supposedly inspire in Newton?

    --That the force of gravity which causes objects to fall was also the force that kept the Moon in its orbit around Earth.

    What did Newton have to assume, to test his guess?

      (1) That the gravitational attraction of a sphere is the same as what we would get, if we let its entire mass be concentrated at the center. By symmetry, of course, the force is directed to the center.

      (2) That the strength of the force of gravity decreased with distance R from the center of attration like 1/R2 ("An inverse square law of attraction").

    Then go through the calculation.

        The French military engineer Augustine Coulomb lived a century after Newton. In 1777 he won a prize for a new method to measure the Earth's magnetism--by suspending a horizontal bar magnet by its middle, from a long string, with a pointer attached--or better, a small mirror (draw on the board). When a beam of light is reflected from the mirror, even small magnetic variations can be measured, even those that twist the magnet by just a tiny amount from its quiet position.

        Coulomb also adapted this "torsion balance" instrument to measure the force between two magnetic poles--either repelling or attracting. You bring the pole end of a second bar magnet near one of the poles of the suspended magnet. The force between them can then be measured by the twisting of the suspension string, which exerts an elastic force. Coulomb found that magnetic poles, like gravity, attracted and repelled with a force that decreased like the inverse of the square of the distance.

        Coulomb's experiment was so sensitive that static electricity, which generated its own forces, sometimes interfered with its observations. That gave Coulomb the idea of measuring electric forces the same way. He replaced the magnet with a little dumb-bell shaped stick, with small spheres at the end. He then charged one sphere with static electricity, brought close to it another charged sphere, and measured the force. Guess what? Electric forces also decreased like 1/R2.

        A reclusive English gentleman named Henry Cavendish set out to use the same method to measure the force of gravity directly. Instead of two lightweight spheres at the end of the stick, he used two heavy ones, and attracted one of them by a third sphere, a big one. The force was tiny and because of the big masses, the instrument reacted agonizingly slowly. But in 1796, he finally succeeded--showing that not just the sphere of the Earth, but even spheres of lead in the lab, exerted a measurable force of gravity.

        Thus for a while it seemed that three major forces in Nature--magnetic, electric and gravity--all obeyed the inverse squares law. Some differences existed--magnetic poles always came in pairs, and unlike the other two forces, which could both attract and repel, gravity could only attract. Nature seemed remarkably symmetric--until 1820, when Oersted and Ampére showed that magnetism was really quite, quite different.

    (Optional Extension).

    Newton's formula for the gravitational attraction between mass m and mass M, at a distance r, is

    F = G M m / r2

    G ("big gee") is known as the gravitational constant, but Newton did not know that number. All he knew was the acceleration g ("little gee") at the Earth's surface, about 9.81 m/s2     What is the relation between G and g ?

      --The weight of mass m is mg Newtons, and that should be equal to the pull of the Earth, if its mass M were concentrated in its center. Let R be the radius of the Earth. Then

      mg = G M m / r2
      g = G M / r2

       G was measured by Hentry Cavendish, using a very refined version of a method developed for measuring electric and magnetic forces (if interested, see section on Coulomb's torsion balance in "A Millennium of Geomagnetism"). It is a very difficult measurement, because G is so small: by the above formulas, if you put m = M = 1 kilogram, r = 1 meter, you find that G is essentially the gravitational pull (in Newtons) between two masses of 1 kg, separated by 1 meter. That is a tiny, tiny force.

    Problem: If the mass density of Earth is D kg/meter3, express g (1 meter3 of water weighs 1000 kg, so D will amount to a few thousands).

      The volume of a sphere of radius R is (4π/3)R3, so its mass M is (4π/3)DR3, and we get

      g = G (4π/3)DR3 / R2 = (4π/3) G D R
        Suppose next that the density D of the Earth was constant throughout its volume, but its radius shrank to 1/4. What would this do to the gravity on the surface of this "smaller earth"?
          There would be less mass pulling down objects on the surface--but the surface would also be closer to the center of attraction! Still, as the above formula shows, the first factor outweighs the second.

      By the formula, if RR/4, then gg/4

      Gravity would decrease to 1/4 of its existing strength.

        The mass of the Moon (derived from its effect on the Earth's motion in space) is 0.012307 times the mass of the Earth, while its radius (measured by telescope) is 0.2725 Earth radii.

        How does g', the gravitational acceleration on the surface of the Moon, compare to g, the acceleration at the surface of Earth? In other words, how would the weight mg' of an object on the Moon compare to its weight mg on Earth? (Use a calculator, of course.)

      The weight of mass m on the Moon is
                        mg' =   G (0.012307 M) m /(0.2725 R)2
                              =   [0.012307/(0.2725)2] G M m / R2
                              =   (0.012307/0.74256) mg   =   0.16574 mg

      Divide both sides by 0.16574

      6.034 g' = g
    So g at Earth is about 6 times stronger.

    Note to the teacher: Variations of this problem can be given as homework or on tests, with numbers adjusted to apply to other planets or moons of the solar system (see Bill Arnett's "nineplanets" web page), or you can invent your own. For instance: what is the gravitational acceleration on the surface of a planet which has twice the radius of Earth and 8 times the Earth's mass? (No need for a calculator with this one.)

        Just based on the preceding calculations, would you say which would have the larger average density--Earth, Moon, both about the same, or we lack enough information to judge?
          Since the Moon's radius is about 1/4 that of Earth (0.2725, more exactly), if both had the same density, then by a previous calculation, g' would be about 4 times weaker than g. Actually, however, g' is smaller than that, about 6 times weaker than g, suggesting the Moon has less density and therefore less attraction.

          Part of the reason may be the iron core of the Earth, since iron is denser than rock. Also, the Earth is much larger, pressing down with greater weight on its innermost layers and perhaps compressing them to greater density.

        Of course, you can also calculate the density, dividing mass by volume, both of which are given here for both Earth and Moon. (If you only want to compare two numbers, the units are not important, though you need more information to get D in kg/meter3.)

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    Author and Curator:   Dr. David P. Stern
         Mail to Dr.Stern:   stargaze("at" symbol)phy6.org .

    Last updated: 10-22-2004